Angular dependence of the resistively-determined irreversibility line in YBCO

We present a wide set of resistive measurements taken in a YBCO thin film, as a function of the temperature, magnetic field and field orientation theta with respect to the (a,b) planes. We focus the attention on the low portion of the resistance curves. Contrary to what is found close to T-c, no angular scaling is possible. We examine the angular behavior of the irreversibility line, conventionally taken as the field and angle dependent temperature T-i (H, theta) at which the resistivity exceeds some sensitivity threshold. It is found that T-i does not follow a simple 2D or 3D behavior, and that its angular dependence is affected by the extended defects present in the film (twin planes) and by the layered structure. We also show that the irreversibility line as a function of the angle, taken at constant reduced field h = H/H-c2 (H-c2 is the temperature-and angle-dependent upper critical field, as determined from the fluctuation conductivity), presents a noticeable angular dependence.